How does the Urinary System Work?

How does the Urinary System Work?

Before you start reading this, do yourself a favor and get a glass of water—you’ll want one before it’s over.

Today we’re talking about the kidneys and that means we’re talking about fluid balance, urine, and one of the detoxification methods for our blood.

The Urinary System

The basic function of the urinary system is to filter our blood before it enters the heart. The toxins that the kidney filters out leave the body through urination.

Unlike many of our other body systems, this one is pretty straight forward. The whole thing consists of two kidneys, two ureters, a bladder, and a urethra. Each kidney is connected to a ureter, both ureters drain into the bladder, the bladder holds fluid (urine) and drains out the urethra—tada, that’s how you pee!

The urethra is is 1 to 1.5 inches in women and up to 8 inches in men and carries urine from the bladder out of the body. The male urethra carries both sperm and urine.

The kidneys are fed blood directly from the aorta, they filter the blood and then pushes clean blood through the vena cava and right back into the heart—if you’re not sure about that path, you can read our article about how the cardiovascular system works.

They filter 24 hours a day, 7 days a week—with just about 200 quarts of fluid passing through every day. On average, about 2 quarts of fluid are removed from the body through urine every day—2 quarts = 8 cups, that’s where we got the “drink 8 cups of water” a day idea (which, by the way, isn’t scientifically proven).


The star players in the kidney world are their nephrons. Up to million of these little buddies live inside a single kidney, and they’re the major detoxifiers and fluid balancers in this system—ie. They make urine.

If you remember when we talked about alveoli in the lungs and how they end in capillaries where gas exchange happens, the method in the nephrons is similar. There are two sections of the nephrons: the renal corpuscle and the renal tubule.

Blood enters the renal corpuscle, which ends in (essentially) a capillary pouch called glomerulus. Here, the blood and fluid are filtered and separated. Blood is taking through the arterial highways back to the heart, and fluid is pushed into the renal tubule.

The magic really happens in the tubule because this is where toxins are pulled out, minerals and excretions are filtered, and water is added or reabsorbed. It’s in this process that the body balances the amount of water in your system along with the pH and composition of things in your bloodstream. The things that are filtered out, are sent with water through the ureters and into the bladder to be removed in your urine.

Nephrons are super sensitive—they’re built to be able to sniff out toxic materials and filter it. The downside of that, is that they are not able to be regenerated in our bodies. We have about 2 million of them, but as we age, or if we’re exposed to a heavy toxic dose, our nephrons are impacted and can die out—making our kidneys much less effective (re: this is when you need dialysis)

Kidneys, Water Balance, and Blood Pressure

Notice that water and minerals are also reabsorbed in the tubule. This is to maintain balance in the body. We’re made of 75% water and it’s up to your kidneys to create that balance for you. If you’re dehydrated, your kidneys will signal to hold on to more fluid (hello waterweight) and if you’re drinking too much it can flush vital minerals from your system (this is less common, but possible)

Not only are you less able to flush out unwanted minerals and toxin from your bloodstream, you could be impacting your blood pressure.

One of the many balancing acts that happens in the kidney (there are a lot, but this is a big one) is between potassium and sodium in the bloodstream, and this often comes down to sodium levels. Sodium is transferred through the body by water (saline) so reabsorbing sodium, means reabsorbing water and vice-versa.

If you have too much sodium in your blood, your heart releases a chemical (atrial natriuretic peptide) that tells your kidneys to pull sodium out of the blood in the filtration process. The release of this hormone decreases blood pressure in the arteries by reducing the amount of water (and thus volume of blood) coursing through your body.

If you don’t have enough sodium in the bloodstream, the adrenal cortex releases a hormone (aldosterone) to tell the kidneys to reabsorb the sodium to create a balance. This hormone increases blood pressure because when the sodium is reabsorbed, so is water and thus, the volume of blood is increased along with your blood pressure.

This balance is finely tuned and is always shifting a little here and there, that’s completely normal.

However, and this is where your diet and digestive system function comes into play, our bodies don’t create sodium on their own to make that balance, so we need to get it from our diets. All too often people eat a prolonged diet too high in sodium, which is known to cause high blood pressure—and this is why. When the digestive system breaks down the sodium and puts it into the bloodstream, the kidneys can’t filter it all out. The increase in sodium increases water in the blood, which increases its volume and with it, the pressure. Aside from blood-pressure medications, people are often placed on diuretic drugs to entice their bodies to remove water, and thus sodium, from their bodies.

The reverse is also true (which not enough sodium, there isn’t enough pressure in the blood), but much less common in our society.

Water and Urine Challenge

This week’s challenge may feel a little strange, but I promise it’s a good one. There a little bit of legwork to it, but it might offer an insight to your kidney health and allow you to get acquainted with your body. The idea is to figure out the right amount of water for you to drink.

Starting with the glass of water you got at the start of the article, write down how much water you drink throughout your day—you can use tallies or any kind of measurement you want for this. Don’t make any changes yet, just keep track of how much.

In the morning, the first time you pee before you flush, look at it. Remember that you’re probably urinating into water, so it’s less concentrated in the bowl than how it came out of you. (You could collect it in a jar for this exercise, I’ve done it, it’s a helpful learning tool).

We look at the first of the day because it tends to be the longest between urinations and so the most concentrated. What are you seeing?

You should see a bit yellow, clear, and without a noticeably strong smell. There isn’t such a thing as the “perfect pee,” but the more you watch your first morning flush, the more you find that you can tell the norm for your body.

Some common indications you may need to drink more water:

Really yellow and/or a strong smell: You likely could drink more water. The color and the smell indicate that the toxins that your kidneys processed are too concentrated.

Bubbles: that’s an indication of internal heat—you could drink more water and work on sensing other types of heat in your body, like a strong/fast pulse, breakouts, strong digestive fire, and work to find a balance.

Cloudy: This is an indication that you need more water but what you’re seeing isn’t always caused by that, these are minerals in your urine. It happens sometimes but if you’re seeing consistent cloudiness in your urine, ESPECIALLY if it’s coupled with any kind of pain, have a checkup with your doctor to make sure you’re not at risk of a serious UTI or kidney stones.

What insight do you hope to gain in your challenge? Tell us in the comments!




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